Conventionally, lithium ion secondary batteries with high voltage and high energy density have been widely used as a power source for electronic equipment, such as notebook personal computers, cellular phones and audiovisual devices, electric powered tools, and the like. Lithium ion secondary batteries typically use a carbon material capable of absorbing and desorbing lithium as a negative electrode active material. As a positive electrode active material, a lithium-containing composite oxide (e.g., LiCoO2) is typically used.
In recent years, as electronic equipment has been reduced in size and improved in performance, there is an increasing need for lithium ion secondary batteries to have higher capacity and longer life. In addition, as the frequency of use of electronic equipment increases in association with the development of ubiquitous network society, there is a strong need for shortening the charging time.
Conventionally, lowering the charge current has been proposed in order to improve the cycle characteristics of secondary batteries. At present, an active material with higher density is used in secondary batteries in order to improve the capacities thereof. If the charge current for such secondary batteries is raised, the acceptance of lithium ions tends to deteriorate, and as a result, the life of the secondary batteries is shortened. Therefore, lowering the charge current to be equal to or less than a predetermined value is effective in improving the cycle characteristics of secondary batteries. However, lowering the charge current in turn decreases the quantity of electricity that can be stored in a secondary battery per unit hour, which prolongs the charging time as a natural result. With regard to the charging time of secondary batteries, shortening the charging time is needed in various fields, and merely lowering the charge current cannot meet the needs.
Under these circumstances, in order to shorten the charging time without sacrificing the charge/discharge cycle life characteristics of secondary batteries (hereinafter simply referred to as “cycle characteristics”), various charge/discharge methods have been proposed. For example, Patent Literature 1 proposes that a secondary battery be charged at a comparative large current initially, and every time when the battery voltage reaches a predetermined cut-off voltage, the charge current be switched to a lower current and, simultaneously, the cut-off voltage be set to a lower voltage.
Patent Literature 2 proposes that constant-current and constant-voltage charging be performed until the battery voltage reaches a predetermined voltage (4.15 V) close to the rated voltage (4.2 V) of a secondary battery, followed by constant-current and constant-voltage charging at a comparatively small current of 0.2 to 0.5 C. Here, 1 C is a current at which the quantity of electricity corresponding to the nominal capacity of a secondary battery can be charged in one hour. Specifically, 0.2 C is one fifth of 1 C, and 0.5 C is one half of 1 C.
Patent Literature 3 proposes that, in a power source system comprising two sets of assembled battery, one of the assembled battery be configured by using a negative electrode having a lithium intercalation potential which is high and be charged and discharged to a depth of charge of 20 to 80%, thereby to make rapid charging possible.